A stepper motor is controlled by the onboard computer. The computer moves the pantile in and out of an opening that bypasses the throttle blade. This allows the computer to precisely control idle speed when varying loads are placed on the engine. Symptoms of failure include engine stalling, surging, poor idle conditions, and or a check engine light. Check for proper operation with each tune-up. Replace when trouble codes indicate failure. Walker Products began supplying the fuel system needs of the automotive industry in 1946. Today Walker is one of the largest, privately owned manufacturers of fuel system components and engine sensors, and has served the needs of the worldwide automotive aftermarket. We are committed to supplying products manufactured to meet or exceed OEM standards and specifications. This effort is backed by our own Manufacturing, product management and engineering staff to ensure it is maintained and accurate in the production of quality parts, vehicle applications, instruction sheets and technical support. Our products include Carburetor and Fuel Injection Repair Kits, Oxygen Sensors and other Engine Management products such as Throttle Position Sensors, Cam and Crankshaft Sensors, Fuel Injectors, Spark Plug Wires and Gaskets. As an ISO 9001: 2008/TS16949 certified company, we are committed to supplying the highest quality products. Vehicle Fitment: 1997-2001 Ford Expedition 4. 6L, Ford F-150 4. 6L 5. 4L, Ford F-250 4. 6L